# I What differentiates thermal and sound energy?

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1. Aug 8, 2017

### smartypants123

Since thermal energy is the vibration of molecules and sound is too, what is the difference? And why exactly do atoms radiate different energies as a result of vibrations?

2. Aug 8, 2017

### davenn

Since you have marked your thread with an "I" tag, indicating you are a university undergraduate education level.
What have your research efforts shown for you so far ?

3. Aug 9, 2017

### smartypants123

So i learned that sound energy is when molecules vibrate and resonate at a certain frequency, and those frequencies vibrate other atoms all the way to our ears. And I also learned that thermal energy is also the vibration of molecules, and the hotter a substance is, the more it vibrates. So if both energies are very different but come from the same source, how can that be? What differentiates them?

4. Aug 9, 2017

### davenn

no, the source is very different
1) what causes sound in the air or an object? give an example
2) what causes heat in the air or in an object? give an example

OK here's a Q for you

do you know the 3 ways thermal energy propagates ( spreads) ?

Sound energy is created differently and only propagates in one way and it is one of the modes that heat uses

5. Aug 10, 2017

### EspressoDan

6. Aug 10, 2017

### davenn

7. Aug 10, 2017

### Justintruth

The difference is that the thermal energy cannot carry information but sound can.

The reason is that the energy distribution of thermal energy is random and the distributions we call sound energy are not.

"Sound" implies one class of distributions of energy across the particles in a gas and "thermal" implies another, different, class of distributions.

Thermal energy implies that the particles are distributed in space randomly and that the velocity directions are distributed randomly. The velocity magnitudes are distributed consistent with the Boltzmann distribution which is also derived from assumptions that the distribution is random.

Sound energy implies that there are compressions and rarefications of the particles and there is an equation (a wave equation) that describes the distribution and that a small program could use this wave equation to output the positions and velocities of the particle A program that output the random particles position and velocities would have to have each velocity and position written into it so that it would be longer than just the list of positions and velocities.

In this sense "something can be said" in the form of a wave equation for sound but "nothing can be said" of the random particles except that they are random.

With respect to thermal energy, one can actually say that one can carry information with it in a sense. For example if you heat one end of a tube and then cool it and have a variable time between the heating and cooling then the heat can conduct down the tube and be measured at the other end and interpreted as a signal. However the term "thermal" usually means "at equilibrium". So if by "thermal" you mean "at equilibrium" then you cannot carry information. When we heated one end of the tube we took the system out of equilibrium.

If by "thermal" you mean any system not at equilibrium then you have a big problem as it turns out even sound distributions could then be thermal. Sound emerging from thermal energy could occur but it violates the second law of thermodynamics. The other way, sound becoming thermal, happens all the time when a sound "dies down". Sound distributions actually become "thermal" distributions when the chaotic dynamics of the air "randomizes" the distribution. Thus a sound wave in a gas will eventually result in a gas that is "thermal" but at a higher temperature. The initial gas with the sound wave in it had some of its energy in thermal energy and some in sound energy but the sound energy becomes thermal energy adding increasing or "heating" the thermal energy, which in systems with positive heat capacity causes an increase in temperature.

8. Aug 10, 2017

### smartypants123

I'm sorry and I know your trying to be helpful in this case but can you just answer my question rather than replying with some of your own?

9. Aug 10, 2017

### smartypants123

thanks for the detailed explanation. I think I get it now. (:

10. Aug 10, 2017

### davenn

the idea of that is to help you learn .... if you just get spoonfed everything you learn nothing and so that isn't the way we work here at PF
We help the person to help themselves by giving hints and posing further questions

Anyway the response @Justintruth didn't really get you along the path as intended and he /she needs to learn not to respond like that

you really should try and answer the questions I posed so that you can work it out a little better to see the clear differences

11. Aug 10, 2017

### smartypants123

OK so sound is caused when all the atoms/molecules making up an object vibrate at the same amount with the same energy. An example would be a metal spoon being hit with another metal spoon, making a sound which sends off a frequency to our ears. Heat is caused by random vibrations, which means that not all the particles in an object would be vibrating and they are less consistent. particles with these vibrations have higher amplitude rather than frequency and that's why our ears cannot hear heat but can only hear sound.

I'm not sure about the three ways for thermal energy to propagate but I honestly don't really care. I was able to answer your other questions because of the other people who commented of this thread and who actually tried directly approaching the question. I appreciate you trying to take a teachers approach, but I was just trying to find out about the difference between sound and thermal energy, which the others really helped out on. Plus if I was able to answer all your questions and was able to see a clear difference between sound and thermal energy then I wouldn't be asking this question. You basically responded to my question with a few more which were of the same concept (which was unfamiliar to me). And if your saying that @Justintruth is wrong, can you just answer to question fully? because his/her explanation seemed pretty logical and unless you give me a reason to not believe it then that's what I will do.

12. Aug 12, 2017

### Wooten

Thanks fore the helpful and detailed explanation Justin!